Gas turbine engine dual towershaft accessory gearbox assembly with a transmission

ABSTRACT

A gas turbine engine assembly includes a turbine section having first and second turbines mounted for rotation about a common rotational axis within an engine static structure, first and second turbine shafts coaxial with one another and to which the first and second turbines are respectively operatively mounted, first and second towershafts respectively coupled to the first and second turbine shafts, an accessory drive gearbox mounted to the engine static structure, the accessory drive gearbox including a first gear train and a second gear train, the first towershaft extending into a housing and coupled to the first gear train, the second towershaft extending into the housing and coupled to the second gear train, a hydraulic pump, a transmission coupling the hydraulic pump to the first gear train, the transmission transitionable between a first mode where the hydraulic pump is driven at a first speed relative to the first towershaft, a second mode where the hydraulic pump is driven at a different, second speed relative to the first towershaft, and a third mode where the hydraulic pump is driven at a different, third speed relative to the first towershaft.

BACKGROUND

This disclosure relates to an accessory drive gearbox for a gas turbineengine.

A gas turbine engine typically includes a fan section, a compressorsection, a combustor section, and a turbine section. Air entering thecompressor section is compressed and delivered into the combustorsection where it is mixed with fuel and ignited to generate a high-speedexhaust gas flow. The high-speed exhaust gas flow expands through theturbine section to drive the compressor and the fan section. Thecompressor section typically includes low and high pressure compressors,and the turbine section includes low and high pressure turbines.

A typical gas turbine engine utilizes one or more gearboxes to driveaccessory components, such as generators, fuel pumps and oil pumps. Eachof the accessory drive components must be driven at a desired rotationalspeed. As a result, the accessory is coupled to either the low or highspeed spool and geared accordingly to obtain the speed at which theaccessory operates more efficiently. Thus, it is not uncommon to use onegearbox coupled to the low speed spool to drive lower speed accessorydrive components, and use a separate gearbox coupled to the high speedspool to drive the other accessory drive components at a higher speed.

One gearbox has been proposed in which the accessory drive componentsare driven by a single towershaft. Other gearboxes have been proposed inwhich some accessory drive components are driven by a first towershaft,and other accessory drive components are driven by a second towershaft.

SUMMARY

A gas turbine engine assembly according to an exemplary aspect of thepresent disclosure includes, among other things, a turbine sectionhaving first and second turbines mounted for rotation about a commonrotational axis within an engine static structure, first and secondturbine shafts coaxial with one another and to which the first andsecond turbines are respectively operatively mounted, first and secondtowershafts respectively coupled to the first and second turbine shafts,an accessory drive gearbox mounted to the engine static structure, theaccessory drive gearbox including a first gear train and a second geartrain, the first towershaft extending into a housing and coupled to thefirst gear train, the second towershaft extending into the housing andcoupled to the second gear train; a hydraulic pump, and a transmissioncoupling the hydraulic pump to the first gear train, the transmissiontransitionable between a first mode where the hydraulic pump is drivenat a first speed relative to the first towershaft, a second mode wherethe hydraulic pump is driven at a different, second speed relative tothe first towershaft, and a third mode where the hydraulic pump isdriven at a different, third speed relative to the first towershaft.

In a further non-limiting embodiment of the foregoing assembly, thetransmission is further transitionable to at least one fourth mode wherethe hydraulic pump is driven at a fourth speed relative to the firsttowershaft, the fourth speed different than each of the first, second,and third speeds.

In a further non-limiting embodiment of any of the foregoing assemblies,the first and second turbine shafts are inner and outer shafts,respectively, and the first and second turbines are low and highpressure turbines, respectively.

In a further non-limiting embodiment of any of the foregoing assemblies,the second towershaft is configured to rotate at a higher speed than thefirst towershaft.

In a further non-limiting embodiment of any of the foregoing assemblies,a first set of accessories are configured to be rotationally driven bythe first towershaft through the first gear train, and a second set ofaccessories are configured to be rotationally driven by the secondtowershaft through the second gear train.

In a further non-limiting embodiment of any of the foregoing assemblies,the first set of accessories includes the hydraulic pump.

In a further non-limiting embodiment of any of the foregoing assemblies,the first set of accessories further includes an oil pump.

In a further non-limiting embodiment of any of the foregoing assemblies,the hydraulic pump is driven at a first speed relative to the oil pumpwhen the transmission in first mode, the hydraulic pump is driven at adifferent, second speed relative to the oil pump when the transmissionis in the second mode, and the hydraulic pump is driven at a different,third speed relative to the oil pump when the transmission is in thethird mode.

In a further non-limiting embodiment of any of the foregoing assemblies,the oil pump is configured to communicate oil to a bearing compartmentincluding bearings supporting the first turbine shaft.

In a further non-limiting embodiment of any of the foregoing assemblies,the first set of accessories further includes an electric machine drivenby the first gear train through the transmission.

In a further non-limiting embodiment of any of the foregoing assemblies,the electric machine is driven at a first speed relative to the oil pumpwhen the transmission in first mode, the electric machine is driven at adifferent, second speed relative to the oil pump when the transmissionis in the second mode, and the electric machine is driven at adifferent, third speed relative to the oil pump when the transmission isin the third mode.

In a further non-limiting embodiment of any of the foregoing assemblies,the second set of accessories includes at least one of a hydraulic pump,a fuel pump, an air turbine starter, and a permanent magnet alternator.

In a further non-limiting embodiment of any of the foregoing assemblies,the first and second gear trains include first and second sets of gearsrespectively coupled to the first and second towershafts, the first setof gears is not in meshing engagement with the second set of gears.

In a further non-limiting embodiment of any of the foregoing assemblies,the first gear train and the second gear train arranged within a commonhousing.

A method of driving accessories through an accessory gearbox of a gasturbine engine according to an exemplary aspect of the presentdisclosure includes, among other things, rotating a first turbine shaftto drive a first towershaft, and rotating a second turbine shaft todrive a second towershaft, the first and second turbine shaft mountedfor rotation about a common rotational axis within an engine staticstructure, driving a first gear train of an accessory drive gearbox withthe first towershaft, and driving a second gear train of the accessorydrive gearbox with the second towershaft, driving a transmission withthe first gear train to rotate an hydraulic pump at a first speedrelative to the first towershaft when the transmission is in a firstmode, transitioning the transmission to a second mode, driving thetransmission with the first gear train to rotate the hydraulic pump at adifferent, second speed relative to the first towershaft, transitioningthe transmission to a third mode, and driving the transmission with thefirst gear train to rotate the hydraulic pump at a different, thirdspeed relative to the first towershaft.

In a further non-limiting embodiment of the foregoing method, the methodincludes driving an oil pump with the first gear train at a first speedrelative the first towershaft when the transmission is in the firstmode, when the transmission is in the second mode, and when thetransmission is in the third mode.

In a further non-limiting embodiment of any of the foregoing methods,the method includes supplying oil to a bearing compartment with the oilpump, the bearing compartment including bearings supporting the firstturbine shaft.

In a further non-limiting embodiment of any of the foregoing methods,the method includes driving an electric machine with the first geartrain through the transmission such that the electric machine is drivenat a first speed relative to the oil pump when the transmission in firstmode, the electric machine is driven at a different, second speedrelative to the oil pump when the transmission is in the second mode,and the electric machine is driven at a different, third speed relativeto the oil pump when the transmission is in the third mode.

In a further non-limiting embodiment of any of the foregoing methods,the first and second turbine shafts are inner and outer shafts,respectively, and the first and second turbines are low and highpressure turbines, respectively, wherein the second towershaft isconfigured to rotate at a higher speed than the first towershaft.

In a further non-limiting embodiment of any of the foregoing methods,the method includes transitioning the transmission to at least onefourth mode, and driving the transmission with the first gear train torotate the hydraulic pump at least one fourth speed relative to thefirst towershaft, the fourth speed different than the first, second, andthird speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be further understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 schematically illustrates a gas turbine engine embodiment.

FIG. 2 is a schematic view illustrating a common accessory drive gearboxdriven by both high and low speed spools.

FIG. 3 is a perspective view of the accessory drive gearbox with theaccessory drive components mounted thereto.

FIG. 4 shows a schematic view of a portion of the accessory drivegearbox of FIG. 3 coupled to an electric machine through a transmissionin a first mode.

FIG. 5 shows a schematic view of a portion of the accessory drivegearbox of FIG. 3 coupled to the electric machine through a transmissionin a second mode.

FIG. 6 shows a schematic view of a portion of the accessory drivegearbox of FIG. 3 coupled to the electric machine through a transmissionin a second mode.

The embodiments, examples and alternatives of the preceding paragraphs,the claims, or the following description and drawings, including any oftheir various aspects or respective individual features, may be takenindependently or in any combination. Features described in connectionwith one embodiment are applicable to all embodiments, unless suchfeatures are incompatible.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. Alternative engines mightinclude an augmenter section (not shown) among other systems orfeatures. The fan section 22 drives air along a bypass flow path B in abypass duct defined within a nacelle 15, while the compressor section 24drives air along a core flow path C for compression and communicationinto the combustor section 26 then expansion through the turbine section28. Although depicted as a two-spool turbofan gas turbine engine in thedisclosed non-limiting embodiment, it should be understood that theconcepts described herein are not limited to use with two-spoolturbofans as the teachings may be applied to other types of turbineengines including three-spool architectures.

The exemplary gas turbine engine 20 generally includes a low speed spool30 and a high speed spool 32 mounted for rotation about an enginecentral longitudinal axis X relative to an engine static structure 36via several bearing systems 38. It should be understood that variousbearing systems 38 at various locations may alternatively oradditionally be provided, and the location of bearing systems 38 may bevaried as appropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects a fan 42, a first (or low) pressure compressor 44 and afirst (or low) pressure turbine 46. The inner shaft 40 is connected tothe fan 42 through a speed change mechanism, which in exemplary gasturbine engine 20 is illustrated as a geared architecture 48 to drivethe fan 42 at a lower speed than the low speed spool 30. The high speedspool 32 includes an outer shaft 50 that interconnects a second (orhigh) pressure compressor 52 and a second (or high) pressure turbine 54.A combustor 56 is arranged in the exemplary gas turbine engine 20between the high pressure compressor 52 and the high pressure turbine54. A mid-turbine frame 57 of the engine static structure 36 is arrangedgenerally between the high pressure turbine 54 and the low pressureturbine 46. The mid-turbine frame 57 further supports bearing systems 38in the turbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis X which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and fan drive gear system 48 may be varied. For example,gear system 48 may be located aft of combustor section 26 or even aft ofturbine section 28, and fan section 22 may be positioned forward or aftof the location of gear system 48.

The gas turbine engine 20 in one example is a high-bypass gearedaircraft engine. In a further example, the gas turbine engine 20 bypassratio is greater than about six (6), with an example embodiment beinggreater than about ten (10), the geared architecture 48 is an epicyclicgear train, such as a planetary gear system or other gear system, with agear reduction ratio of greater than about 2.3 and the low pressureturbine 46 has a pressure ratio that is greater than about five. In onedisclosed embodiment, the gas turbine engine 20 bypass ratio is greaterthan about ten (10:1), the fan diameter is significantly larger thanthat of the low pressure compressor 44, and the low pressure turbine 46has a pressure ratio that is greater than about five 5:1. Low pressureturbine 46 pressure ratio is pressure measured prior to inlet of lowpressure turbine 46 as related to the pressure at the outlet of the lowpressure turbine 46 prior to an exhaust nozzle. The geared architecture48 may be an epicycle gear train, such as a planetary gear system orother gear system, with a gear reduction ratio of greater than about2.3:1. It should be understood, however, that the above parameters areonly exemplary of one embodiment of a geared architecture engine andthat the present invention is applicable to other gas turbine enginesincluding direct drive turbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the gas turbine engine 20is designed for a particular flight condition—typically cruise at about0.8 Mach and about 35,000 feet (10,668 meters). The flight condition of0.8 Mach and 35,000 ft (10,668 meters), with the engine at its best fuelconsumption—also known as “bucket cruise Thrust Specific FuelConsumption (‘TSFCT’)”—is the industry standard parameter of lbm of fuelbeing burned divided by lbf of thrust the engine produces at thatminimum point. “Low fan pressure ratio” is the pressure ratio across thefan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The lowfan pressure ratio as disclosed herein according to one non-limitingembodiment is less than about 1.45. “Low corrected fan tip speed” is theactual fan tip speed in ft/sec divided by an industry standardtemperature correction of [(Tram ° R)/(518.7° R)]^(0.5). The “Lowcorrected fan tip speed” as disclosed herein according to onenon-limiting embodiment is less than about 1150 ft/second (350.5meters/second).

Referring to FIG. 2 with continuing reference to FIG. 1, the gas turbineengine 20 includes an accessory drive gearbox 60, or gearbox, that isrotationally driven by both the inner and outer shafts 40, 50. Theaccessory drive gearbox 60 includes a housing 62 within which first andsecond sets of gears 64, 66 are arranged. A first towershaft 68 extendsthrough a strut 69 to interconnect the inner shaft 40 to the first setof gears 64. The first towershaft 68 can drive the first set of gears 64through a layshaft, for example. A second towershaft 70 extends througha strut 71 to interconnect the outer shaft 50 to the second set of gears66. The second towershaft 70 can drive the second set of gears 66through a layshaft, for example. In another example, the firsttowershaft 68 could interconnect to the outer shaft 50 and the secondtowershaft 70 could interconnect to the inner shaft 40.

The example accessory drive gearbox 60 is mounted to, or adjacent to, anintermediate case 74 of the gas turbine engine 20. In some examples, thefirst and second towershafts 68, 70 extend through the intermediate case74, and the housing 62 of accessory drive gearbox 60 is mounted to theintermediate case 74. The housing 62 is thus downstream from the firstand second towershafts 68, 70 relative to a general direction of flowthrough the engine 20. The struts 69, 71 can be part of the intermediatecase 74 and used to connect the intermediate case 74 to other portionsof the gas turbine engine 20.

In some embodiments, the first and second sets of gears 64, 66 arearranged within the housing 62 in close proximity to reduce an overallsize of the accessory drive gearbox 60. Sizing the accessory drivegearbox 60 in this way may permit the first and second towershafts 68,70 to extend through the struts 69, 71 and have the struts 69, 71directly circumferentially adjacent to each other such that no otherstrut of the intermediate case 74 is positioned circumfernetiallybetween the struts 69, 71. In this example, the first and second sets ofgears 64, 66 are arranged in a common housing. Other example could housethe first and second sets of gears 64, 66 in separate housings.

Placing the first and second towershafts 68, 70, within adjacent strutson the intermediate case 74 can reduce the effects of thermal expansiondifferences between the accessory drive gearbox 60 and the gas turbineengine 20 The temperature of the intermediate case 74 can vary inresponse to the temperature of the engine gaspath, which can changerelatively rapidly in response to power settings. The temperature of theaccessory drive gearbox 60 is, in contrast to the intermediate case 74,is kept fairly constant by a coolant, such as oil The temperaturedifference between the intermediate case 74 and the accessory drivegearbox 60 can cause different amounts of thermal expansion. Groupingthe first and second towershafts 68, 70 relatively closely together canreduce an overall thermal growth of the accessory drive gearbox 60relative to the intermediate case 74. Further, packaging space withinthe nacelle 15 of the gas turbine engine 20 is limited and often notconducive to incorporating multiple, separate accessory drive gearboxesin separate housings.

Referring now to FIG. 3, with continuing reference to FIGS. 1 and 2, theexemplary accessory drive gearbox 60 utilizes the second set of gears 66to rotatably drive a fuel pump 80, an air turbine starter 84, and apermanent magnet alternator 88. The second towershaft 70 is driven bythe outer shaft 50 to rotatably drive the second set of gears 66.

The exemplary accessory drive gearbox 60 utilizes the first set of gears64 to rotatably drive an oil pump 96 and a transmission 100. An electricmachine 104 and a hydraulic pump 108 are rotatably driven by the firstset of gears 64 through the transmission 100. The first towershaft 68 isdriven by the inner shaft 40 to rotatably drive the first set of gears64.

In an exemplary non-limiting embodiment, the transmission 100 is athree-speed transmission that can transition between a first mode ofshown schematically in FIG. 4, a second mode shown schematically in FIG.5, and a third mode shown schematically in FIG. 6.

In the first mode, the transmission 100 is rotated by the firsttowershaft 68 through the first set of gears 64 and, in response,rotates the electric machine 104 and the hydraulic pump 108 at a firstratio relative to a rotational speed of the first towershaft 68. In thesecond mode, the transmission 100 is rotated by the first towershaft 68through the first set of gears 64 and, in response, rotates the electricmachine 104 and the hydraulic pump 108 at a different, second ratiorelative to a rotational speed of the first towershaft 68. In the thirdmode, the transmission 100 is rotated by the first towershaft 68 throughthe first set of gears 64 and, in response, rotates the electric machine104 and the hydraulic pump 108 at a different, third ratio relative to arotational speed of the first towershaft 68.

During operation, the inner shaft 40 can experience a greater range ofrotational speeds that the outer shaft 50. That is, the speed excursionfor the inner shaft 40 can be higher than the speed excursion for theouter shaft 50. In a specific non-limiting embodiment, the inner shaft40 can operate at speed excursions of up to 80% during operation of thegas turbine engine 20, whereas the outer shaft 50 can operate at speedexcursions of up to 30% during operation of the gas turbine engine 20.

In this exemplary embodiment, the first towershaft 68 is geared to theinner shaft 40, such that the rotational speed of the first towershaft68 and the first set of gears 64 varies linearly with the rotationalspeed of the inner shaft 40. Also, the second towershaft 70 is geared tothe outer shaft 50 so that the rotational speed of the second towershaft70 and the second set of gears 66 varies linearly with the rotationalspeed of the outer shaft 50.

The transmission 100 addresses issues associated with rotating theelectric machine 104 and the hydraulic pump 108 with a rotatable inputfrom the first towershaft 68 from the inner shaft 40. In the exemplaryembodiments, the transmission 100 operates in the first mode when theinner shaft 40 is rotating at a speed excursion of, say, less than 25%.If the speed excursion of the inner shaft 40 meets or exceeds 25% but isless than 50%, the transmission 100 switches to the second mode torotate the electric machine 104 and the hydraulic pump 108. If the speedexcursion of the inner shaft 40 meets or exceeds 50%, the transmission100 switches to the third mode to rotate the electric machine 104 andthe hydraulic pump 108.

This permits the first set of gears 64 to drive the electric machine 104and the hydraulic machine 108 through the transmission 100 at threedifferent ratios. The electric machine 104 is thus not required tooperate across a range of speed excursions from 0 up to 80% duringoperation of the gas turbine engine 20. Instead, due to the transmission100, the range is no more than 30% for the electric machine 104 and thehydraulic machine 108. The electric machine 104 and the hydraulicmachine 108 can operate more efficiently when the electric machine 104is rotated across a smaller range of rotational speeds than across alarger range of rotational speeds.

An electronic engine control (EEC) 112 can control the transition of thetransmission 100 between the first mode, the second mode, and the thirdmode. The EEC 112 could, for example, receive an input corresponding tothe rotational speed of the inner shaft 40, and then transition thetransmission 100 from the first mode to the second mode or the thirdmode when the rotational speed exceeds a threshold speed.

Although the exemplary transmission 100 can transition between threemodes, other exemplary embodiments of the transmission could transitionbetween more than three modes. In such examples, the transmission isrotated by the first towershaft 68 through the first set of gears 64and, in response, rotates the electric machine 104 and the hydraulicpump 108 at four or more different ratios relative to a rotational speedof the first towershaft 68

In this example, the electric machine 104 is a variable frequencygenerator that receives a rotational input to generate power utilized bycomponents of the gas turbine engine 20. Other examples couldincorporate other types of generators, and other types of electricmachines. The variable frequency generator is rated at 90 kVA in someexamples. The transmission 100 facilitates incorporating the variablefrequency generator rather than, for example, an integrated drivegenerator since the transmission 100 permits operating the electricmachine 104 in a narrower rpm range while still being driven by rotationof the inner shaft 40 and the towershaft 68.

The hydraulic pump 108 generally moves hydraulic fluid needed to movecomponents of an air frame to which the gas turbine engine 20 ismounted. The transmission 100 permits operating the hydraulic pump 108to be driven in a narrower rpm range while still being driven byrotation of the inner shaft 40 and the towershaft 68. Driving thehydraulic pump 108 with the inner shaft 40 rather than the outer shaft50 can improve engine operability and performance. The exhaust gastemperature is also reduced as there is less power draw on the outershaft 50.

The oil pump 96 is driven at a fixed ratio relative to the speed of thefirst towershaft 68. That is, switching the transmission 100 between thefirst and second modes does not substantially change a ratio ofrotational speeds between the first towershaft 68 and the oil pump 96.Thus, as the rotational speed of the first towershaft 68 varies, therotational input to the oil pump 96 varies linearly with the rotationalspeed of the first towershaft 68.

In this example, the oil pump 96 is dedicated to supplying oil to thenumber 1 bearing systems 38′, which incorporates thrust bearingsdirectly supporting the inner shaft 40. The thrust bearings are taperedbearings in some examples.

The rotational speed of the first towershaft 68 increases when therotational speed of the inner shaft 40 increases. The inner shaft 40 mayrequire additional lubrication, such as oil, directed to bearing systems38 supporting the inner shaft 40 when the rotational speed of the innershaft 40 increases.

The increased lubrication demands due to increasing the rotational speedof the inner shaft 40 are met by increasing the rotational input speedto the oil pump 96. In other words, the amount of oil moved to thenumber 1 bearing system 38′ varies linearly with the rotational speed ofthe inner shaft 40. If the oil pump 96 were instead varying linearlywith the rotational speed of the outer shaft 50, the oil pump 96 maymove more oil than is required for lubrication. The excess oil wouldneed to recirculated, or accommodated in some other way, which resultsin losses.

The oil pump 96 is considered a 60% oil pump as it accommodatesapproximately 60% of the lubrication requirements for the gas turbineengine 20. An additional pump, not shown, such as an electric pump,could be incorporated into the engine and powered by the electricmachine 104 to supply lubricant to other areas of the gas turbine engine20.

The fuel pump 80 is utilized to supply fuel to the gas turbine engine 20during start-up, and during other stages of operation. Thus, the fuelpump 80 is driven by rotation from the outer shaft 50. The outer shaft50 spins up as the engine is started prior to the inner shaft 40. Thatis, rotation of the low speed spool 30 lags rotation of the high speedspool 32. Thus, pumping fuel with the fuel pump 80 is required duringinitial rotation of the outer shaft 50.

In some examples, the fuel pump 80 could be driven electrically withpower from the electric machine 108 rather than being rotated by theouter shaft 50 through the first set of gears 66. In such an example,the power extraction for the fuel pump is effectively moved to the innershaft 40 and reduce power extraction from the outer shaft 50.

The air turbine starter 84 is utilized during start-up and thusconfigured to be driven by the outer shaft 50. Also, the permanentmagnet alternator 92 can be used to power a full authority digitalelectronics control (FADEC), which can include the EEC 108. As the FADECis used during start up, the permanent magnet alternator 92 is alsodriven by the outer shaft 50.

Referring again to the oil pump 96, an added feature of couplingrotation of the oil pump 96 with rotation of the inner shaft 40 is thatthe inner shaft 40 spins with the fan 42. Thus, during a windmillingevent when the fan 42 is spinning without being driven by the innershaft 40, the oil pump 96 can continue to pump oil lubricating thebearings associated with the inner shaft 40. If the oil pump 96 weredecoupled from rotation with the inner shaft 40, another pump or anelectronic pump could be required to move oil to lubricate the fan 42when windmilling.

Features of the some of the disclosed examples include driving accessorycomponents with the low speed spool of a gas turbine engine rather thana high speed spool. This can reduce power draw on the high spool andthereby reduce an exhaust gas temperature. The low speed spool can alsowork without rotation of the high speed spool, which can facilitatepowering some accessory components, such as an oil pump, when the engineis windmilling.

It should also be understood that although a particular componentarrangement is disclosed in the illustrated embodiment, otherarrangements will benefit herefrom. Although particular step sequencesare shown, described, and claimed, it should be understood that stepsmay be performed in any order, separated or combined unless otherwiseindicated and will still benefit from the present invention.

Although the different examples have specific components shown in theillustrations, embodiments of this invention are not limited to thoseparticular combinations. It is possible to use some of the components orfeatures from one of the examples in combination with features orcomponents from another one of the examples.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of the claims. For that reason, the following claimsshould be studied to determine their true scope and content.

What is claimed is:
 1. A gas turbine engine assembly comprising: aturbine section having first and second turbines mounted for rotationabout a common rotational axis within an engine static structure; firstand second turbine shafts coaxial with one another and to which thefirst and second turbines are respectively operatively mounted; firstand second towershafts respectively coupled to the first and secondturbine shafts; an accessory drive gearbox mounted to the engine staticstructure, the accessory drive gearbox including a first gear train anda second gear train, the first towershaft extending into a housing andcoupled to the first gear train, the second towershaft extending intothe housing and coupled to the second gear train; a hydraulic pump; anda transmission coupling the hydraulic pump to the first gear train, thetransmission transitionable between a first mode where the hydraulicpump is driven at a first speed relative to the first towershaft, asecond mode where the hydraulic pump is driven at a different, secondspeed relative to the first towershaft, and a third mode where thehydraulic pump is driven at a different, third speed relative to thefirst towershaft.
 2. The gas turbine engine assembly of claim 1, whereinthe transmission is further transitionable to at least one fourth modewhere the hydraulic pump is driven at a fourth speed relative to thefirst towershaft, the fourth speed different than each of the first,second, and third speeds.
 3. The gas turbine engine assembly of claim 1,wherein the first and second turbine shafts are inner and outer shafts,respectively, and the first and second turbines are low and highpressure turbines, respectively.
 4. The gas turbine engine assembly ofclaim 3, wherein the second towershaft is configured to rotate at ahigher speed than the first towershaft.
 5. The gas turbine engineassembly of claim 3, wherein a first set of accessories are configuredto be rotationally driven by the first towershaft through the first geartrain, and a second set of accessories are configured to be rotationallydriven by the second towershaft through the second gear train.
 6. Thegas turbine engine assembly of claim 5, wherein the first set ofaccessories includes the hydraulic pump.
 7. The gas turbine engineassembly of claim 6, wherein the first set of accessories furtherincludes an oil pump.
 8. The gas turbine engine assembly of claim 7,wherein the hydraulic pump is driven at a first speed relative to theoil pump when the transmission in first mode, the hydraulic pump isdriven at a different, second speed relative to the oil pump when thetransmission is in the second mode, and the hydraulic pump is driven ata different, third speed relative to the oil pump when the transmissionis in the third mode.
 9. The gas turbine engine assembly of claim 7,wherein the oil pump is configured to communicate oil to a bearingcompartment including bearings supporting the first turbine shaft. 10.The gas turbine engine assembly of claim 8, wherein the first set ofaccessories further includes an electric machine driven by the firstgear train through the transmission.
 11. The gas turbine engine assemblyof claim 10, wherein the electric machine is driven at a first speedrelative to the oil pump when the transmission in first mode, theelectric machine is driven at a different, second speed relative to theoil pump when the transmission is in the second mode, and the electricmachine is driven at a different, third speed relative to the oil pumpwhen the transmission is in the third mode.
 12. The gas turbine engineassembly of claim 5, wherein the second set of accessories includes atleast one of a hydraulic pump, a fuel pump, an air turbine starter, anda permanent magnet alternator.
 13. The gas turbine engine assembly ofclaim 1, wherein the first and second gear trains include first andsecond sets of gears respectively coupled to the first and secondtowershafts, the first set of gears is not in meshing engagement withthe second set of gears.
 14. The gas turbine engine assembly of claim 1,wherein the first gear train and the second gear train arranged within acommon housing.
 15. A method of driving accessories through an accessorygearbox of a gas turbine engine, comprising: rotating a first turbineshaft to drive a first towershaft, and rotating a second turbine shaftto drive a second towershaft, the first and second turbine shaft mountedfor rotation about a common rotational axis within an engine staticstructure; driving a first gear train of an accessory drive gearbox withthe first towershaft, and driving a second gear train of the accessorydrive gearbox with the second towershaft, driving a transmission withthe first gear train to rotate an hydraulic pump at a first speedrelative to the first towershaft when the transmission is in a firstmode; transitioning the transmission to a second mode; driving thetransmission with the first gear train to rotate the hydraulic pump at adifferent, second speed relative to the first towershaft; transitioningthe transmission to a third mode; and driving the transmission with thefirst gear train to rotate the hydraulic pump at a different, thirdspeed relative to the first towershaft.
 16. The method of claim 15,further comprising driving an oil pump with the first gear train at afirst speed relative the first towershaft when the transmission is inthe first mode, when the transmission is in the second mode, and whenthe transmission is in the third mode.
 17. The method of claim 16,further comprising supplying oil to a bearing compartment with the oilpump, the bearing compartment including bearings supporting the firstturbine shaft.
 18. The method of claim 16, further comprising driving anelectric machine with the first gear train through the transmission suchthat the electric machine is driven at a first speed relative to the oilpump when the transmission in first mode, the electric machine is drivenat a different, second speed relative to the oil pump when thetransmission is in the second mode, and the electric machine is drivenat a different, third speed relative to the oil pump when thetransmission is in the third mode.
 19. The method of claim 15, whereinthe first and second turbine shafts are inner and outer shafts,respectively, and the first and second turbines are low and highpressure turbines, respectively, wherein the second towershaft isconfigured to rotate at a higher speed than the first towershaft. 20.The method of claim 15, further comprising transitioning thetransmission to at least one fourth mode, and driving the transmissionwith the first gear train to rotate the hydraulic pump at least onefourth speed relative to the first towershaft, the fourth speeddifferent than the first, second, and third speeds.